Delivering medications via contact lenses has been a prevailing practice since the inception of using hydrophilic, crosslinked polymer gels on the surface of the eye. In fact, the first patent in the field from Otto Wichterle in 1965 states that “bacteriostatic, bacteriocidal or otherwise medicinally active substances such as antibiotics may be dissolved in the aqueous constituent of the hydrogels to provide medication over an extended period, via diffusion.” However, there is evidence that this notion of a dissolved component in an aqueous constituent has been around for a much longer period of time. Evidence exists that honey soaked linen was used in ancient Rome as an ophthalmic dressing in the treatment of disease.
The biggest obstacle to using the fluid entrained in the aqueous portion of the polymer gel is maintaining a significant concentration of drug within the fluid to have a therapeutically relevant effect, which is ultimately limited by the solubility of the drug. This has been the primary reason why drug release from contact lenses has not become a clinical or commercial success. To an equivalent extent, the control over the drug delivery profile and an extended release profile is also important to therapeutic success and has not been demonstrated using these methods. Drug uptake and release by conventional (i.e., currently available) soft contact lenses can lead to a moderate intraocular concentration of drug for a very short period of time, but does not work very well due to a lack of sufficient drug loading and poor control of release. The use of soft, biomimetic contact lens carriers (i.e., recognitive polymeric hydrogels) described herein has the potential to greatly enhance ocular drug delivery by providing a significant designed and tailorable increase in drug loading within the carrier as well as prolonged and sustained release with increased bioavailability, less irritation to ocular tissue, as well as reduced ocular and systemic side effects.
The ocular bioavailability of drugs applied to the eye is very poor (i.e., typically less than 1-7% of the applied drug is absorbed with the rest entering the systemic circulation). Factors such as ocular protective mechanisms, nasolacrimal drainage, spillage from the eye, lacrimation and tear turnover, metabolic degradation, and non-productive adsorption/absorption, etc., lead to poor drug absorption in the eye. Currently, more efficient ocular delivery rests on enhancing drug bioavailability by extending delivery and/or by increasing drug transport through ocular barriers (e.g., the cornea—a transparent, dome-shaped window covering the front of the eye; the sclera—the tough, opaque, white of the eye; and the conjunctiva—a mucous membrane of the eye with a highly vascularized stroma that covers the visible part of the sclera). A topically applied drug to the eye is dispersed in the tear film and can be removed by several mechanisms such as:                (i) irritation caused by the topical application, delivery vehicle, or drug which induces lacrimation leading to dilution of drug, drainage, and drug loss via the nasolacrimal system into the nasopharynx and systemic circulation (e.g., the rate drainage increases with volume);        (ii) normal lacrimation and lacrimal tear turnover (16% of tear volume per minute in humans under normal conditions);        (iii) metabolic degradation of the drug in the tear film;        (iv) corneal absorption of the drug and transport;        (v) conjunctival absorption of the drug and scleral transport;        (vi) conjunctival “non-productive” absorption via the highly vascularized stroma leading to the systemic circulation; and        (vii) eyelid vessel absorption leading to systemic circulation.Therefore, due to these mechanisms, a relatively low proportion of the drug reaches anterior chamber ocular tissue via productive routes such as mechanisms (iv) and (v).        
For posterior eye tissue and back of the eye diseases (e.g., age-related macular degeneration, retinal degeneration, diabetic retinopathy, glaucoma, retinitis pigmentosa, etc.), the amount of drug delivered can be much less compared to front of the eye disease. To treat back of the eye disease, four approaches have typically been used, topical, oral (systemic delivery), intraocular, and periocular delivery.
Topically applied drugs diffuse through the tear film, cornea/sclera, iris, ciliary body, and vitreous before reaching posterior tissues, but due to the added transport resistances such diffusions do not typically lead to therapeutically relevant drug concentrations. However, researchers have shown that topically applied drugs do permeate through the sclera by blocking corneal absorption and transport. Intravitreal injections (injections into the eye) require repeated injections and have potential side effects (hemorrhage, retinal detachment, cataract, etc.) along with low patient compliance. Extended release devices have been used but require intraocular surgery and often have the same incidence of side effects. Periocular drug delivery is less invasive and also requires injections or implant placement for predominantly transscleral delivery.
To overcome most of these protective mechanisms, topical formulations have remained effective by the administration of very high concentrations of drug multiple times on a daily basis. For a number of drugs high concentrations can lead to negative effects such as burning, itching sensations, gritty feelings, etc., upon exposure of the medication to the surface of the eye as well as increased toxicity and increased ocular and systemic side effects. However, traditional ophthalmic dosage forms such as solutions, suspensions, and ointments account for 90% of commercially available formulations on the market today. Solutions and suspensions (for less water soluble drugs) are most commonly used due to the ease of production and the ability to filter and easily sterilize them. Ointments are used to a much lesser extent due to vision blurring, difficulty in applying to the ocular surface, and greasiness. The term “eye drops” herein refers to all topological medications administered to a surface of the eye, including but not limited to solutions, suspensions, ointments and combinations thereof. In addition to the aforementioned problems, drug delivery through the use of eye drops does not provide for controlled time release of the drug. Eye drop medications typically have a low residence time of the drug on the surface of the eye.
The efficacy of topical solutions has been improved by viscosity enhancers that increase the residence time of drugs on the surface of the eye, which ultimately lead to increased bioavailability as well as more comfortable formulations. Also, inclusion complexes have been used for poorly soluble drugs, which increase solubility without affecting permeation.
Other recent delivery methods have included in situ gel-forming systems, corneal penetration or permeation enhancers, conjunctival muco-adhesive polymers, liposomes, and ocular inserts.
Ocular inserts, in some cases, achieve a relatively stable or constant, extended release of a drug. For example, ocular inserts such as Ocusert® (Alza Corp., FDA approved in 1974) consist of a small wafer that contains a drug reservoir enclosed by two ethylene-vinyl acetate copolymer membranes. The wafer is placed in the corner of the eye and provides extended release of a therapeutic agent for approximately 7 days (e.g., pilocarpine HCL, for glaucoma treatment, reducing intraocular pressure on the eye by increasing fluid drainage). Lacrisert (Merck) is a cellulose based polymer insert used to treat dry eyes. However, inserts have not found widespread use due to occasional noticed or unnoticed expulsion from the eye, membrane rupture (with a burst of drug being released), increased price over conventional treatments, etc.
Mucoadhesive systems and in-situ forming polymers typically have problems involving the anchorage of the carrier as well as ocular irritation resulting in blinking and tear production. Penetration enhancers can cause transient irritation, or alter normal protection mechanisms of the eye, and some agents can cause irreversible damage to the cornea.